1. Field of the Invention
This invention relates to a heat resisting alloy especially excellent in cold workability and suitable to be used for exhaust valves of automotive engines, and a method for producing the exhaust valves using the above-mentioned heat resisting alloy.
2. Description of the Prior Art
Heretofore, as a material for an exhaust valve of automotive engines or so, high Mn austenitic heat resisting steel JIS SUH 35 (Fe-9Mn-21Cr-4Ni-0.5C-0.4N) or Ni-based super alloy JIS NCF 751 (Ni-15.5-Cr-0.9Nb-1.2Al-2.3Ti-7Fe-0.05C) has been used, for example.
Although the aforementioned Ni-based super alloy is an alloy excellent in high-temperature strength, high-temperature oxidation resistance and high-temperature corrosion resistance, there is a problem in the cost since the alloy contains expensive Ni as much as a little more than 70 wt %.
Accordingly, an approach for decreasing the amount of expensive Ni has been carried out, and alloys containing Ni of 40 wt % or less have been already developed.
However, further decrease of Ni content causes problems in properties of the alloy and it is difficult realistically to further reduce the Ni content in the alloy.
Namely, if the Ni content is further decreased, stability of the structure at a high-temperature is detriorated owing to increase of Fe, .eta.-phase (Ni.sub.3 Ti) which is a brittle phase is precipitated during the long time application at a high-temperature, thereby bringing deterioration in the high-temperature strength and the toughness at a room temperature of the alloy. Thus, there is a limit naturally in the decrease of the Ni content for a reason of the problem in the properties of the alloy.
On the other side, an exhaust valve of automotive engines has been produced conventionally through a process of the following steps.
First of all, the heat resisting alloy of Fe--Cr--Ni type is cold-drawn to form a bar with predetermined dimensions. Next, a head portion of the valve is formed through hot upsetting after preforming the bar through electric upsetting, for example. Subsequently, the head portion is usually joined in one body with a stem portion made of a martensitic heat resisting steel specified as JIS SUH-11 (Fe-1.5Si-8.5Cr-0.5C) or SUH-3 (Fe-2Si-11Cr-1Mo-0.4C) through friction welding, for example.
After this, solid solution treatment is performed in order to release a strain stored through the aforementioned workings, and high-temperature strength as the exhaust valve is obtained by precipitating .gamma.'-phase such as (Ni,Cr).sub.3 (Al,Ti,Nb,Ta) through aging treatment.
Furthermore, a tail end of the stem of the valve is hardened by quenching according to demand, and the exhaust valve is forwarded finally after finishing by machining.
However, there is a problem in the aforementioned conventional method in that it is necessary to form the head portion through two steps consisting of the preforming by the electric upsetting after the preforming. The head portion formed through the hot upsetting is not so excellent in the dimensional accuracy and it is required to completely remove defects on a surface of the formed head portion, therefore there are the other problems in that cutting amount in the finishing process is apt to increase and times required for the finishing is also apt to become longer. Furthermore, solid solution treatment is necessary before the aging treatment.
All of these problems are causes for increase of the production cost and are required to be solved.
The former problem in these problems is able to be solved fundamentally by forming the head portion of the valve through a step of cold working or warm working instead of the hot working such as the hot upsetting. In this case, the heat resisting alloy as a raw material of the head portion is required to be excellent in the cold workability.
However, the heat resisting alloys proposed and used practically up to the present have been developed assuming the hot working in all cases, therefore it has been difficult to form the head portion by carring out the cold or warm forging to the raw material of these alloys.